Method for reducing tangential force variation in pneumatic tires

ABSTRACT

A method and apparatus for reducing tangential force variation in a pneumatic tire wherein the tire is mounted on a wheel rim, inflated to operating pressure, and rotated under a predetermined load against a loading drum. Force transducers on the axis of the loading drum measure the tangential force variation as the tire rotates. These measurements are placed into the memory of a computer. The computer processes this data to obtain the magnitude of the tangential force variation and the phase angle from an arbitrary location of the variation or the angular displacement to the point of maximum magnitude. The computer outputs a V-shaped ramp voltage level signal for controlling the servo-valve which moves a rotary grinder into grinding engagement with the inner tread ribs of the tire. The grinding is repeated for a number of revolutions depending upon the magnitude of the variation.

This is a division, of application Ser. No. 614,381 filed Sept. 18, 1975and now U.S. Pat. No. 4,128,969 dated Dec. 12, 1978.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to post-cure processing of pneumatic tires inorder to obtain optimum uniformity, and specifically to reductions offorce variations in the tangential or running direction.

2. Description of the Prior Art

Due to non-uniformities in tire construction, it has been observed thatextraneous forces and force variations are produced by the tire when thetire is rotating under load. Non-uniformities in the construction of thetire create moments and forces which can have an adverse effect upontire ride and comfort.

Certain of these force variations are well known. The reduction of forcevariations in the radial direction is disclosed in U.S. Pat. No.3,724,137. In radial force variation correction, the tire is mounted ona wheel rim, inflated to normal pressure, and rotated under apredetermined load against a loading drum. Radial force and radial forcevariation are measured on the loading drum by force transducers locatedin the radial direction on the axis of the loading drum. A pair ofrotary grinders positioned adjacent the shoulder of the tire tread aremoved into grinding engagement with the tread shoulder ribs inaccordance with the radial force variations detected on the drum. Theserotary grinders remove material from the shoulder ribs so that the tirebecomes more uniform and the radial force variations are reduced toacceptable levels.

Due to the slowness of the response of the electromagnetic servo-systemwhich controls the movement of the grinders, the tire is rotated at afairly low speed. Typically, the tire is rotated at 60 rpm. This speedis sufficient to detect radial and lateral force variations since suchvariations exist independent of the speed at which the tire is rotated,in the absence of resonances.

It has been found that certain ride disturbances occur as a result oftire non-uniformities deriving from tangential or traction forces, orthose forces parallel to the wheel plane in the direction of motion ofthe tire. These ride disturbances are evident with tires operating atall speeds. Automobiles equipped with radial or other types of tireshave shown such ride disturbances at speeds between 0 and 80 mph, evenwith tires of minimal radial and lateral force variationcharacteristics. These ride disturbances take the form of a vehicleshake--generally a vibration felt once per wheel revolution beinginfluenced by vehicle "wheel hop", and other vehicle resonances.

Tangential force variations or torque variations are speed dependent andmay be generated by a change in angular acceleration of the tire whichoccurs over a portion of the circumference of the rotating tire. Speedsgreater than 60 rpm and possibly in the order of 300 rpm or greater,equivalent to about 30 mph, are necessary for such tangential forcevariations to be accurately detected.

Current methods and apparatus for improving tire uniformity do notprovide for reduction of tangential force variations but are ratherconcerned primarily with the reduction of radial force variations.

SUMMARY OF THE INVENTION

The method and apparatus of the present invention, however, provide forcorrection or reduction of adverse tangential force variations (toreduce ride disturbances). The ride disturbances are primarilyvibrations heard and/or felt by the driver.

One of the objects of the present invention is to improve the uniformityof pneumatic tires.

Another object of the invention is to improve the post-cure processingof pneumatic tires to achieve optimum conditions of tire uniformity.

Yet another object is to improve the ride of pneumatic tires by reducingshake and roughness disturbances.

A further object is to sense and measure tangential force variations andto reduce those variations to acceptable levels.

A still further object is to improve the tire uniformity correctionprocess by providing for an additional function of reducing tangentialforce variations in the tire as it is rotated in a tire uniformitycorrection machine.

Still another object is to provide a method for rapidly reducingtangential force variations automatically on a production-line highspeed tire uniformity machine.

These and other objects and advantages are achieved by the unique methodand apparatus of the present invention wherein a pneumatic tire ismounted on a rim, inflated, and rotated under a predetermined loadagainst a loading drum. As the tire is rotated, force transducersoperatively associated with the drum located on the drum axle in thetangential direction sense and measure tangential force variations. Thevoltage output from these force transducers is fed into a computer whichcalculates the magnitude of the tangential force variations and thephase angle from a reference point on the tire or the angulardisplacement to the point on the tire at which the variations are at amaximum. In accordance with these calculations, the computer outputs aV-shaped ramp voltage level signal for controlling a servo-valve thatmoves a rotary grinder into grinding engagement with the inner ribs ofthe tire tread. The grinder removes material from these inner tread ribsso that the tire is made more uniform and the tangential forcevariations are reduced. Grinding is continued for successive revolutionsuntil the tangential force variations are reduced to acceptable levels.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a tire uniformity machineadapted to practice the method of the present invention;

FIG. 2 is a diagram of a side elevation of a pneumatic tire illustratingthe corrective grinding accomplished to reduce the tangential forcevariations;

FIG. 3 is a graph illustrating the voltage level signal which representsthe composite tangential force variations produced as the tire rotates;

FIG. 4 is a graph illustrating the first harmonic of the signal of FIG.3; and

FIG. 5 is a graph illustrating the voltage level control signal producedby the computer in response to the signal of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring more particularly to the drawings and initially to FIG. 1,there is shown by a schematic representation a typical tire uniformitymachine modified to practice the method of the present invention. A tire10 is mounted and inflated on a rim, such as a solid or split rim 12 oran ordinary car wheel, carried on an axle 14 turned by a variable-speedrotary drive means (not shown). The drive means is capable of rotatingthe tire at speeds typical of conventional tire uniformity machines,such as 24 or 60 rpm for measuring radial and lateral force variationsand conicity. In addition, the drive means should have sufficient powerto rotate the tire at a higher speed. A speed of at least 300 rpm andpreferably at least 800 rpm is necessary in order to produce accuratelymeasurable tangential force variations. Both speeds can be achieved witha dual-speed motor or with a separate motor added to a conventionalmachine with a clutching or similar device.

A loading drum 18 having a circumferential surface is mounted for freerotation on a non-rotating axle 20. Loading means, such as a pair ofhydraulic cylinders 22 connected to the drum axle 20, are used to movethe loading drum 18 into contact with the tire 10 and to apply apredetermined deflecting load. A pair of load cells 24 are mounted onthe drum axle 20. In conventional tire uniformity machines, the loadcells 24 contain sensors, such as strain gauges with flexures, whichmeasure forces in the radial and lateral direction. In order to measuretangential forces, the load cells 24 should also have sensors mounted tomeasure the forces on the drum in the tangential direction, or thehorizontal direction in which the tire is rolling. These sensors producea voltage level signal representative of the tangential force variation.Although, tangential force sensors on the drum axle 20 are preferred,the tangential force variations may be measured in other ways such as bya torque measuring device between the drive means and the tire.

The output of the tangential sensors in the load cells 24 is fed into acomputer 26. Preferably, the computer 26 is a digital computer of theminicomputer class. A minicomputer is preferred due to its widespreadavailability and relative inexpensiveness. However, this does notrestrict the use of other devices, such as those of an analog nature. Aswill be explained in greater detail hereinafter, the computer 26receives and interprets the tangential force variation signal, andstores this information in its memory. Later, the computer produces acontrol signal for controlling the grinding of the tire in accordancewith the tangential force variation signal.

The computer 26 outputs the control signal on line 28. This signal isfed through a servo-amplifier 30 to actuate a servo-valve 32. Theservo-valve 32 moves the rotary grinder 34 into grinding engagement withthe inner ribs of the tread of the tire 10. A special grinding wheel orother cutting device of proper dimensions to touch each of the innertread ribs is used to remove the tread material from the inner or riderribs of the tire. A narrower grinder which moves laterally across thetread ribs could also be used. The outer or shoulder ribs of the tiretread are not touched by the center rib grinder, unlike the correctivegrinding done to reduce radial and lateral force variations. One or moreof the inner ribs may be ground individually as well as all of the innerribs.

The method of the prevent invention involves rotating the tire at twodifferent speeds. Two speeds are required because tangential forcevariations reach measurable levels only when the tire is rotated at highspeeds, but it is not practical to operate the rotary grinder at highspeeds due to the relative slowness of response of the grinder controlelectromagnetic servo-system. Therefore, the tire 10 is first rotated ata higher speed to measure the tangential force variations. Thesemeasurements are stored in the memory of the computer 26. The tire isthen run at a lower speed, at which time the computer 26 outputs theproper control signal to the servo-valve 32 to move the grinder 34 intoengagement with the tire 10 to grind to reduce the tangential forcevariations.

The specific steps of the preferred method of the present invention areas follows. First, a tire 10 is mounted in the rim 12 and inflated, aspreviously described. The tire is rotated against the loading drum 18under a predetermined load, and the radial force variations are reducedusing conventional methods. The speed of the tire 10 is then increasedto a high speed. In order to properly detect the tangential forcevariations, the high speed should be greater than 300 rpm whichcorresponds to a vehicle speed of about 30 mph. Preferably, the drum isrotated at a speed of 840 rpm, corresponding to a tire speed of about 70mph. Tangential force variations are detected by the load cells 24 andthe voltage level signals representing these force variationmeasurements are fed into the computer 26. A typical tangential forcevariation signal is illustrated in FIG. 3.

The computer 26 is programmed to accomplish Fourier analysis on thetangential force variation signal to determine the harmonics of thetangential force variations. With the availability of low-costprogrammable minicomputers, this has been found to be the preferredmethod of obtaining the first harmonic of the tangential forcevariation. However, other known methods of obtaining the first harmonicsignal may also be used. For instance, a suitable first harmonic filtermay be employed, such as a filter of 36 db per octave or similarcharacteristics with a cutoff frequency at the tire rotational speed.For a tire rotating at 600 rpm, such a filter would cut off at about 10Hz and attenuate the signal 36 db at 20 Hz. Several available commercialharmonic analysis devices could be used as well. The phase displacementof the signal by the harmonic analysis device must be compensated for inall cases.

The first harmonic of a tangential force variation is preferably used tocorrectly grind the tire. FIG. 4 shows the first harmonic signalcorresponding to the composite tangential force variation signal of FIG.3. First harmonic force correction is preferred because the firstharmonic is the major cause of the undesirable shake. However, thepresent method could also be applied to the composite force signal, anyhigher harmonic, or the inverse function of several harmonics. These aremajor causes of tire roughness.

The computer 26 compares the maximum magnitude of the first harmonicwith a predetermined acceptable level. If the maximum magnitude is lessthan the acceptable level, no correction is needed. The tire issatisfactory and ready for use, and it is removed from the machine. Ifthe magnitude exceeds the acceptable level, the procedure to reduce thetangential force variations is performed.

In addition to the magnitude of the first harmonic of the tangentialforce variation, the computer also determines the phase angle. The"phase angle" is defined as the angular displacement in degrees from anarbitrary location on the tire to the position of the maximum positivemagnitude of the first harmonic of the tangential force variations. Anexample of a phase angle is shown in FIG. 4 On the tire of FIG. 2, thearbitrary location from which the phase angle is measured is at the topof the tire and is designated 0°. In this example, the phase angle ofthe tire of FIG. 2 is 120°. The first harmonic signal of FIG. 4corresponds to the tire of FIG. 2. The phase angle or angular distanceto the position of maximum positive magnitude of the first harmonicsignal of FIG. 4 is also 120°.

After the tangential force variations have been measured at the highspeed and the magnitude and phase angle of the first harmonic have beenstored in the memory of the computer 26, the speed of the tire 10 isreduced to a slower speed. A typical speed for production tirecorrection is 60 rpm. This speed is fast enough for efficient productionactivity and slow enough to accommodate the response times of theservo-system. At the slow speed, the computer 26 is programmed toproduce a servo-control signal from the data stored therein.

The servo-control signal produced by the computer is a V-shaped rampfunction. FIG. 5 illustrates the control signal produced in response tothe first harmonic signal of FIG. 4. The peak of this V-shaped rampcorresponds to the phase angle of the first harmonic of tangential forcevariation. The ramp begins at a point 90° before the location of thepeak, and the ramp ends at a point 90° after the location of the peak.In the example of FIG. 5 where the phase angle is 120°, the ramp beginsat the point of 30°, increases to the point of 120° and then decreasesto zero at the point of 210°.

The control signal is output by the computer 26 onto line 28. Thissignal is put through the servo-amplifier 30 to actuate the servo-valve32 and bring the rotary grinder 34 into grinding engagement with theinner rib or ribs of the tread of the tire 10. The V-shaped rampcombined with the practical aspects of grinding rubber with a grindingwheel results in a good approximation of a sine wave. The rotary grinder34 grinds off excessive rubber along the inner ribs of the tread inaccordance with the tangential force variations measured at the highspeed. The grinder engages the tire from a point 90° before the locationof the positive maximum magnitude of the tangential force variation to apoint 90° after the location of the positive maximum.

In the example of FIG. 2 where the phase angle is 120°, grinding isbegun at the point on the tire corresponding to 30°. The grinder doesnot touch the tire between the 0° point and the 30° point. The grinderjust touches the tire at the 30° point and goes successively deeper intothe tire until it reaches the point of maximum positive magnitude of thefirst harmonic, corresponding to 120°. Maximum cutting is accomplishedat the 120° point. From the 120° point to the point corresponding to210°, the grinder retracts successively so that at the 210° point it isjust touching the tire. From the 210° point back to the 0° point thegrinder does not touch the tire.

It is not necessary to follow the free radial run-out pattern of thetire as is done in the reduction of radial force variations. Propergrinding can be obtained by setting the grinder adjacent to, but nottouching, the inner ribs of the tire treads, and causing the grinder tomove in the proper distance in accordance with the measured tangentialforce variations.

The grinding or cutting process is repeated during successiverevolutions with the number of revolutions being determined by themagnitude of the force to be reduced. The grinder 34 is then retracted,and the tire 10 is speeded up to the high speed and new values of thetangential force variations are measured. If the values of the forcevariations are now acceptable, the tire is removed from the machine. Ifthe values are beyond the acceptable limits, the process is repeated.Speed up for measurement and slow down for correction may be repeated asmany times as desired. Also, for each type and design of tire, a "table"could be placed in the computer memory which would tell the number ofgrinding revolutions to reduce a specified force variation.

With high speed uniformity machines under computer control, this processcan be carried out very rapidly. It is estimated that only a few secondsare necessary to correct each tire. Thus the method of the presentinvention may be applied to production tire corrections.

It is possible to measure and correct at the same speed. The limitationlies in the relative slowness of the response of the grinder controlelectromagnetic servo-system. However, a properly designed grindersystem and the proper choice of servo-system components or equivalentsystems may permit measurement and reduction at some higher speed atwhich tangential force variations may be measurable.

Other modifications and variations in the specific method herein shownand described will be apparent to those skilled in the art all withinthe intended scope and spirit of the invention. While the invention hasbeen shown and described with respect to a specific embodiment thereof,this is intended for the purpose of illustration rather than limitation.Accordingly, the patent is not to be limited to the specific embodimentherein shown and described nor in any other way that is inconsistentwith the extent to which the progress in the art has been advanced bythe invention.

We claim:
 1. In a method for reducing tangential force variations in apneumatic tire, which comprises the steps of:(a) rotating the tire at aspeed greater than 300 rpm against a predetermined load; (b) sensing andmeasuring the force variations in the tangential direction while thetire is rotating; (c) determining the magnitude of the tangential forcevariations and the location of maximum magnitude of tangential forcevariations; (d) comparing said magnitude to a predetermined level todetermine if the force variations are within acceptable limits; whereinthe improvement is characterized by: (e) grinding rubber only from theinner ribs of the tire at a reduced tire rotational speed at thelocation of maximum magnitude of the tangential force variations if thetangential force variations exceed acceptable limits.